Liquid crystalline monomers and mixtures of monomers in combination with photoinitiators can exhibit nematic mesophases. Under ultraviolet ("UV") irradiation these nematic mesophases can undergo rapid photopolymerization, in the presence of UV-photoinitiators, to freeze-in the structure and orientation (alignment) of the nematic mesophase into a polymer matrix.
For instance, Japanese 62 70,406 describes the polymerization of nematic monomers in the liquid crystalline state in the presence of a polymerization initiator with ultraviolet radiation. One or more of the monomers are heated to melt and then cooled into their nematic mesophase. The mesophase is aligned and photopolymerized to obtain an aligned polymer film. The monomers disclosed have the following structure: ##STR2## wherein n is 2, 3, 4, 5, 6, 8 or 11, X is -CN, H, Cl, etc., R is CH.sub.3 or H, and Y is covalent bond or --CO.sub.2 --.
It has been found that compositions disclosed in the cited patent are of limited usefulness in photopolymerization processes because single monomers or mixtures of the monomers disclosed undergo rapid crystallization from the monotropic nematic phase, thus destroying the desired oriented nematic structure before photopolymerization can be performed. Therefore, it is desirable to have polymerizable nematic monomer mixtures that resist crystallization at room temperature.